To anyone who knows even a little about how Wikipedia is edited, this shouldn't be a mystery at all; there are no automatic pages or statistics, even for fixed entities like dates, and there is no automatic cross-referencing.
So as you'd expect, a person entered every birthday on that page, and there is absolutely no reason to assume that comparable detail has been given to all other days of the year.
(I'd hardly call this an easter egg, and I wish more people understood Wikipedia enough to make this entirely uninteresting.)
wikidata[1][2] is used in some places, for example the population of london could be stored in wikidata, and referenced from all the translations of the london article.
Amusingly, Birthdays is the first example given in the wikipedia article about wikidata.
[1]https://www.wikidata.org/wiki/Wikidata:Introduction [2]https://en.wikipedia.org/wiki/Wikidata
It's easier to build a list of notable births from a search and update one page than to go through notable people, finding their birthdate and updating multiple pages.
I found that the page is usually pretty quiet most of the year with a few days around the date it happens with events that year and also "20 years since" articles get published